1. Field of the Invention
The present invention relates to a method for evaluating the fracture toughness using the instrumented indentation testing, and more particularly relates to a method for evaluating the fracture toughness by using the instrumented indentation testing, which enables direct measurement of physical properties in the field so that the structural integrity can be evaluated without separately collecting a specimen, which enables a calculation of the fracture toughness by executing the indentation testing through a theoretical and practical approach in order to thereby identify the resistance to the crack growth in the specimen, and which is useful enough to be used for the evaluation of the physical properties in the field, such as the evaluation of a change in the fracture toughness depending on the temperature or the evaluation of the structural integrity.
2. Description of the Prior Art
Since unexpected breakage of a large structure brings about significant losses, the evaluation of the structural integrity is performed in various manners in order to avoid the breakage. In order to evaluate the integrity of a structure that is in use, it is important to know exact information on the stress that is applied to the structure, information on the defects that exist inside the structure, and physical property information of a structure material. In particular, the breakage of the structure is caused by a change in the physical properties, such as a deterioration or embrittlement, in many cases, so it is required to measure the mechanical properties of a material that is in actual use, instead of the physical properties at the time when it is designed, to then be reflected. However, the conventional mechanical testing cannot be applied to evaluate, in real time, the mechanical properties of a structure that is in use because it is a destructive test in which the mechanical properties of a material are measured by making and breaking a standardized specimen.
The plane strain fracture toughness (JIC) and the elastic-plastic stress intensity factor (KJC), which show the fracture toughness that indicates the resistance to the crack growth of a material, are important dynamic parameters for evaluating the structural integrity. However, the fracture toughness measuring method according to the prior art requires a specimen of a specific shape and size in order to ensure the validity. That is, correct testing is established, in which a crack is formed in a specimen and the stress and strain around the crack are dynamically analysed in order to thereby measure the size and the growing direction of the crack, and thereafter, the conditions for the fracture may be obtained through the testing. The representative fracture toughness test methods include compact tension testing, which introduced fatigue precracking, and 3-point-single-edge notched bend testing. For the testing above, a plurality of specimens having a predetermined size are taken in a predetermined direction from the original plate (see ASTM specifications for the load direction and the crack growth direction), and the crack growth is recognized while actually breaking the same.
However, the complicated testing procedure including the fatigue precracking and the measurement of the crack length causes the fracture toughness measurement to be extremely difficult.
Furthermore, since the conventional fracture toughness measuring method is a destructive test in which a specimen is taken from the material to then be tested, it has limitations in being applied to industrial structures that are actually operated.
In the instrumented indentation testing that is proposed as an alternative to the same, the load and the indentation depth are measured in real time while applying a load to the surface of a specimen by using a sharp indenter, and then, a variety of mechanical properties of the material are evaluated through the analysis of a measured load-depth curve. Unlike the conventional testing methods, since this testing is a non-destructive mechanical test that leaves only a small indentation on the material surface, the physical properties can be directly measured on the spot, and a local property evaluation can be made through the testing in a local area. Therefore, the testing is widely used as micro/nanoscale mechanical testing, such as thin films or electronic components. Recently, a technique has been developed, which evaluates the tensile property, the residual stress, and the fracture toughness in the indentation testing, as well as the basic properties thereof, such as the hardness and the elastic modulus, based on dynamic modelling.
Among them, although the fracture toughness is the advanced physical property that is required for the destructive-dynamic analysis of the structural integrity, unlike the strength that is used in the general structural analysis, the analysis thereof is difficult because of the influences of various parameters (such as the crack length, the specimen shape, or the like) and the standardized fracture toughness testing is also difficult to be applied because of its complicated procedure and the nature of destructive testing. Thus, there have been many studies for evaluating or measuring the fracture toughness by using the indentation testing.
In addition, although it is considered that the confinement effect before the occurrence of the crack in the destructive testing is similar to the confinement effect that occurs under the indenter in the indentation testing in relation to the evaluating method of the fracture toughness by using the conventional indentation testing, in the strict sense, the destructive testing generates a plane-symmetrical stress field in the shape of a cylinder, whereas the indentation testing generates an axis-symmetrical stress field in the shape of a sphere. Accordingly, the fracture toughness may not be accurately evaluated by using the conventional indentation testing.